Abstract

An indicator strip, useful in analytical chemical procedures, which comprises an impregnated capillary material having at least a partially transparent film on one side and on the other side, a film forming a hollow space which is in communication with said capillary material.

ATTORNEYS INVENTORS ADOLF WENZ ALFRED STEIN DIETER SCHMITT WW INDICATOR STRIP BACKGROUND OF THE INVENTION In analytical chemistry procedures, indicating devices permitting a rapid determination of the identity of substances dissolved in a liquid have become of increasing importance. Mainly, the detection procedures are conducted with colorimetric indicating devices, wherein qualitative as well as quantitative determinations are possible. However, especially in connection with the quantitative or semiquantitative detection procedures, the indicating devices known heretofore are less than satisfactory.

Indicating devices are known, for example, which contain capillary-type test papers, enveloped by casings of a synthetic resin material, which change color in the presence of a specific substance in the medium to be examined. Normally, the length of the portion of the capillary material which has been color-changed is a measure of the concentration of the substance to be determined in the examined medium. The intensity of color produced is not considered in most cases in the analytical procedure. It is not possible to use, in this respect, reagents for the indicator portion which are soluble in the medium to be examined, since the reagents will be irregularly distributed in the impregnated layer by the penetration of the medium, and thereby cause a non-uniform coloring of the indicator portion. Test rods have been used which contain one or more test paper indicator strips sealed between plastic films. These rods exhibit a serious disadvantage, because relatively viscous liquid media, such as serum, for ex ample, are very slowly absorbed by the indicator paper. Furthermore, during the absorption of the liquid media, an undesired partial chromatographical separation usually takes place. The reagents contained in the indicator papers, insofar as they are soluble in the liquid to be tested, are also subjected to this chromatographic effect, as well as the components dissolved in the liquid to be tested. This results in a non-uniform color change of the impregnated zone, and it is difficult to be coordinated with a corresponding color scale. The determination is additionally impaired by an air bubble, which normally forms between the indicator paper and the top film when the test rod is immersed into the medium to be examined.

SUMMARY OF THE INVENTION A principle object of this invention is to provide an improved indicator strip useful for qualitative and quantitative analytical chemical procedures, which avoids the disadvantages heretofore mentioned.

Another object of this invention is to provide a novel construction of an indicator strip having at least one side of a reagent-pregnated capillary material in communication with a hollow space.

Another object of this invention is to provide a novel construction of an indicator strip having a partially transparent film on one side of a reagent pregnated capillary material.

Upon further study of the specification and appended claims, other objects and advantages of this invention will become apparent.

To attain these objects, there is provided a novel indicator strip comprising a reagent impregnated capillary material having on one side a partially transparent film and on the other side, a film construction which provides at least one slot or hollow space in communication with the capillary material. The indicator strip is constructed in a variety of forms to provide versatility to various analytical conditions. The main advantages of the construction is to enable the capillary material to absorb the liquid medium to be examined quickly, to prevent chromatographic separation and other problems associated with analytical procedures using heretofore conventional indicator strips.

DETAILED DISCUSSION OF THE INVENTION This invention relates to a novel, improved indicator strip consisting of an absorbent, impregnated capillary material enveloped by at least partially transparent films, wherein the films expose the absorbent capillary material in at least one area, and wherein at least one side of the impregnated capillary material is in communication with at least one hollow space. Thus, an absorbent capillary material, impregnated vvith reagents, can be incorporated between two plastic films in such a manner that one film contacts the capillary material and that a hollow space, which can also be subdivided into several small spaces is formed between the capillary material and the other film. It is also possible to provide the plastic films in such a manner that hollow spaces are formed above as well as below the impregnated capillary material. Furthermore, two impregnated strips of the capillary material can also be disposed between two plastic films in such a manner that the hollow space is produced between the capillary material strips.

Decided advantages are obtained by providing the hollow spaces above the capillary material impregnated withreagents. The impregnated capillary material absorbs the liquid medium to be examined essentially more quickly than is the case in the conventional indicator paper strips. A defined and predetermined volume of the liquid medium is absorbed. Because of the rapid, complete absorption by the impregnated indicator portion, there is neither a chromatographic separation impairing the coloring mode, nor a migration of reagents soluble in the liquid in the capillary material. The impregnated capillary material assumes a characteristic color or color change corresponding to the content of the components in the liquid medium to be tested, and accordingly makes it possible to conduct a quantitative, or at least a semiquantitative, determination. The formation of undesired air bubbles between the capillary material and the plastic film when the indicator is immersed in the liquid to be examined is avoided. Accordingly, the read-off accuracy is increased, and the coordination of the resultant characteristic color or color change with a color scale is essentially facilitated.

Suitable films of a synthetic material are conventional films employed heretofore for such purposes. The films must not be porous, and they must be liquidimpcrmeable. At least one of the films is to be transparent. Suitable materials for the films are, for example, polyethylene, polypropylene, polyvinyl-type polymers, or copolymers, such as polyvinyl chloride, polyvinyl acetate, polyvinylidene chloride, or

polyethylene glycol terephthalates and the like. Also laminated films can be employed if an increased rigidity and a sufficient heat sealing strength of the films is desired. Plastic films coated with fusion adhesives or heat-sealing varnishes are likewise suitable. The thickness of these films can vary between and 1000p, preferably between 50 and 200 n. It is not necessary for both films to exhibit the same thickness. Rather, the film covering the capillary material can, for example, be very thin (e.g., .1.), whereas the film forming the hollow space on the other side of the capillary material can suitably be substantially thicker (up to 1000 p.) so that it exhibits sufficient strength and rigidity. If desired, either or both of the films can also be self-sealing, so that a sealing step can be omitted. In this case, the adhesive must, of course, be selected so that is does not interfere with the detection reactions to be conducted. Alternatively, the films enveloping the capillary material can be welded together according to all customary methods, for example by pressure, heat, or high frequency, or also by the effect of solvents and/or swelling agents. At least one of the films must be transparent. It is often advantageous to make the other film opaque or even colored, since, in this manner, the resultant color effects can sometimes be enhanced, whereby the read-off accuracy is increased.

Any absorbent material can be employed for the capillary material which is to be saturated with reagents. Thus, all porous materials can be utilized which can absorb or take up a liquid or a solid substance. The material should exhibit capillary properties due to a porous structure. Preferred materials are fibrous substances, e.g., paper, especially filter paper and the like, however any other material having a capillary or wicktype effect can also be employed. Some further examples are other cellulose materials, synthetic fibers, glass fibers, finely divided granulated substances, natural and synthetic fibrous meshwork, and the like.

Advantageously, this capillary material is employed in thicknesses of about 0.05 3 mm., preferably about 0.2 0.4 mm. The surface of the capillary material, not covered by plastic films, intended for reaction, preferably has an approximately square configuration and has an area of about between l0 and 50 mm. The capillary material should be uniform with respect to porosity, density, and chemical properties.

The indicator strips of this invention are suitably designed so that the hollow space preferably extends over the entire surface of the impregnated capillary material at least on one side. In any event, at least 40 percent of one surface of the capillary material should be in communication with a hollow space in order to obtain the desired results. The hollow space can also be subdivided into several smaller spaces, and this can also be done either on one or on both sides of the capillary material. In order to permit the liquid, medium to be tested to penetrate and enter into a reaction with the applied reagents on the capillary material, it must be exposed at least on one and preferably on both lateral edges, i.e., the film must not cover the capillary material at those places. However, it is also possible to expose the capillary material on all edges, whereby ventilating is afforded.

If it is desired, in individual cases, to provide additional ventilating, the films can also contain perforations at any desired places. However, in general, such modifications are unnecessary in connection with the embodiments of this invention.

It is important to design the hollow space over the capillary material in such a manner that it exhibits a certain height as compared to the thickness of the capillary material. It was found that especially advantageous indicator strips are obtained if the proportion of the thickness of the capillary material with respect to the maximum height of the hollow space, or spaces, disposed thereabove, as seen in cross section, is from about 1 0.5 to about 1 10, preferably from about 1 l to about 1 2 2. Surprisingly, it is not the volume of the hollow space which is of particular importance, but rather the ratio of thickness to height.

It is also possible to accommodate several different capillary materials, or several capillary materials saturated with different reagents, on one indicator strip, when it is desirable for several investigations to be conducted simultaneously, or in the case, during an examination step, the detection is to be carried out simultaneously by means of different reactions. In this regard, the individual hollow spaces above the capillary materials can optionally be in communication with one another, or they can also be segregated from one another. Thus, in the latter case, a multiple-chamber indicator strip is obtained, and it is preferred if there is the danger that reagents contained in the capillary material, or the colorations which occur, influence one another and thus can impair the determinations. Even in the case where several individually independent reactions are to be conducted by means of a single test strip, this concept is of special advantage. The separation can be obtained either by the shape of the film or by special separating layers. Thus, it is possible, for example, to encompass the capillary material on one or both sides with materials, preferably hydrophobic, e.g., resins, waxes, or synthetic materials. In a multiplechamber indicator strip, the capillary material and the separating layers, optionally can be arranged in an alternating series. Such indicator strips can be very advantageously manufactured if the thickness of the separating layers exceeds the thickness of the capillary material by the desired height of the hollow space above the capillary material. In this way, the separating layers, preferably a synthetic material, function as spacers, since the cover films can then be sealed. or glued directly onto the separating layers extending beyond the capillary material. The width of the separating layers can be selected as desired and can thus be adapted to the respective purpose of the indicator strip.

It is also possible to impregnate two strips of the capillary material with various reagent substances, especially for example, with mutually incompatible substances, and then to provide these strips of capillary material in one indicator strip in such a manner that a common hollow space is produced in the center. One surface of the capillary materials, in each instance, closely contacts the films, whereas, respectively, the other surface faces the common hollow space present in the center. In such an arrangement, both films are preferably transparent.

Reference is now made to the drawings wherein various embodiments of the indicator strips according to the invention are illustrated. The drawings, in order to attain greater clarity, are not drawn to scale.

FIGS. 1-6 show longitudinal sectional views of various embodiments of the indicator strips of the invention;

FIGS. 7a and 7b show cross-sectional views through two embodiments of the invention;

FIGS. 8-13 show longitudinal sectional views of additional embodiments of the indicator strips of the invention;

FIG. 14 is a perspective view of the embodiment shown in FIG. 5;

FIG. shows eight variations (a through h) of film designs for providing a series of smaller spaces next to the capillary material of the indicator strips; and

FIG. 16 is a top view of another embodiment of the indicator strip of the invention showing two variations of the inner surface of the film of the indicator strip.

In all the embodiments shown in the drawings, the numeral 1 denotes the capillary material, 2 denotes one of the films, 3 denotes the cover film, 4 denotes the slot or hollow space above the capillary material, and 5 denotes projections, bulges, curved sections, peaks, and the like in the film 3.

In FIG. 1, the capillary material 1 of the indicator strip is sealed or glued to the film 2 on one side, whereas the cover film 3 forms the slot or hollow space 4 above the capillary material 1. Near the upper and lower edges of the capillary material 1, the films 2 and 3 are welded or sealed together.

The indicator strip shown in FIG. 2 corresponds essentially to that in FIG. 1. The film 3 is designed to contain a small shoulder, in such a manner that the capillary material 1 is sealed to the film 3 and it is not attached to the film 2 in this instance.

In FIG. 3, the film 3 is provided with projections or protuberant portions 5, which can subdivide the hollow space 4 into various smaller cavities if the projections 5 contact the capillary material 1.

In FIG. 4, another embodiment is disclosed for subdividing the hollow space 4 into various smaller spaces. In this case, the cover film 3 is of serpentine form and can contact or firmly touch the capillary material 1, or an interstice between 1 and 3 can be left so that a coherent hollow space is produced as shown.

FIG. 5 shows an embodiment wherein the capillary material 1 is not sealed within the film, but rather it is covered for protection on one side by the film 2, whereas the film 3 forms the required hollow space 4. Thus, according to this embodiment, the capillary material is not only exposed from the side of the edges along the lateral sides, but also from above and from the bottom at the narrow sides.

FIG. 6 corresponds essentially to the modification of FIG. 5, with the difference that here the film 3 is designed so that the hollow space 4 is subdivided, by the projections 5 and into individual smaller spaces when the projections 5 touch the capillary material 1.

FIGS. 7a and 7b represent cross sections through the indicator strips of this invention and in particular show a serpentine film 3 parallel to the longitudinal edges of the indicator strips. Suitably, the lower edges of the capillary material on the'narrow sides of the indicator strips are left free in these embodiments, whereby the capillary effect by the individual hollow cylinders produced above the capillary material is increased.

In FIG. 7a, the capillary material 1 is sealed onto the film 2 and onto the projections 5 of-the film 3, whereas, according to FIG. 7b, the capillary material 1 merely rests on the film 2 and is sealed within the films 2 and 3.

FIG. 8 shows an embodiment of the indicator strips according to the invention wherein a hollow space 4 is provided on both sides of the capillary material 1. The films 2 and 3 are in each case welded together below and above the capillary material 1. Basically, the hollow spaces 4 can exhibit different volumes, but, for the sake of simplicity, symmetrical arrangements will normally be selected.

FIG. 9 represents another embodiment of the indicator strip wherein two, optionally different, strips of the capillary material 1 contact the films 2 and 3, respectively, whereas a common hollow space 4 is formed between the two capillary materials 1. Above and below the capillary material 1, the films 2 and 3 are joined to each other, for example by cementing or welding. In this embodiment, it is advantageous to employ transparent film for films 2 and 3.

In FIGS. 10 and 11, embodiments of indicator strips are illustrated which contain several strips of the capillary material 1. The remaining design corresponds approximately to FIGS. 1 and 2.

FIGS. 12 and 13 show embodiments of indicator strips wherein the capillary material 1 is disposed between hydrophobic separating layers 6. The capillary material 1 and the hydrophobic material 6 are sealed to the film 2. The height of the separating layers 6 determines the height of the hollow space 4. The film 3 is sealed to the separating layers 6. A sealing between the films 2 and 3 can be omitted in this embodiment, wherein the impregnation of the capillary material 1 is especially well protected.

FIG. 14 is a perspective view from the side and the embodiment essentially corresponds to FIG. 5. FIG. 14 is to demonstrate the exposed edges 7 and 8 of the capillary material 1.

FIG. 15 shows, in embodiments (a) through (h), various shaped designs for the film 3. Basically, any shape can be chosen which makes it possible to subdivide the hollow space 4 into various smaller spaces. In this regard, it is unnecessary to provide that the film 3 exhibits the same shape over the entire surface above the capillary material 1. Rather, a multitude of possibilities of variation, as well as all types of transitions conceivable are possible. Accordingly, projections, grooves, curves, peaks, mosaic configurations, etc. can be provided regularly or irregularly, and can either rest on the capillary material 1, or can terminate above the material 1 in the hollow space 4.

FIG. 16 shows, in a top view, an indicator strip according to the invention wherein the inner surface 5 of film 3 can be formed of various patterns. Two types are shown as examples. I

Normally, the film 2 is to be transparent, whereas the film 3 is opaque or colored. The read-off then takes place, if a hollow space is provided only on one side, i.e., on the side which-faces away from this hollow space.

The required shaping of the film 3 can be done in accordance with conventional methods, for example by embossing or deep-drawing. However, it is also possible to apply plastics, powders or granulated'substances by sintering. Also other small particles suitable for this purpose, e.g., glass fibers or minerals and the like, can alter the inner surface of the film 3 in the desired manner by being melted or sprayed thereon.

The novel indicator strips can be successfully used in all cases where a qualitative, or also quantitative determination of substances dissolved or suspended in a liquid test medium is to be conducted. The mode of application does not differ from that of the conventional indicators. Normally, the indicator strips are immersed into the liquid to be tested, the color changes caused after a period of time are noted, and/or compared with a color scale. The speed and the accurate reproducibility attainable by the indicator strip according to the invention represents a considerable advance in this art.

During the manufacture, the impregnated capillary material, preferably paper, is cut to the desired width and incorporated in between the films depending on the design selected. In many cases, it proved to be especially advantageous to seal or cement the strip-shaped capillary material to a film prior to the impregnation step. If a capillary material thus joined to a film is then impregnated, it is generally possible to avoid the bending or curling of the saturated papers after drying, which interferes with the further processing operations. The thus-obtained welded plastic bands are then cut transversely, so that indicator strips are produced which are open from the edges at the narrow sides. The indicator strips normally exhibit a width of about 3 I mm., preferably about 5 7 mm., and a length of about 30 100, preferably about 50 70 mm. However, other dimensions are, of course, likewise possible.

Without further elaboration, it is believed that one skilled in the art, can, using the preceding description, utilize the present invention to its fullest extent. The following preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever.

EXAMPLE I Copper Test Strips A filter paper strip having a width of about 6 mm. and a thickness of about 320 or 370 p. (Schleicher and Schull No. 2316 or 1450 CV") is sealed, approximately 6 mm. from the lower edge, to a transparent film band having a width of about 60 mm. (and a thickness of about 100 u), and is continuously saturated with a solution having the following composition:

0.2 g. of Cupron I g. of hydroxylamine hydrochloride 50 ml. ofmethanol 50 ml. of benzene Thereafter, the band is dried by means of hot air or infra red light. Then, an opaque film having also a width of about 60 mm. and a thickness of about 100 s, exhibiting a bulge of a height of 700 p. according to FIG. 1, is sealed on with the aid ofa pair of rolls (heatable roll and chill roll). The bulge extends in a width of about 6 mm. at a spacing of 5 I2 mm. from the lower edge. The center of the bulge is located approximately over the center of the impregnated capillary material. The thus-sealed film band is reeled up, and the rolls are cut, at right angles to the conveying direction of the film band, into test strips having a length of-60 mm. and a width of about 6 mm. Thereafter, the capillary material is exposed on the cut edges.

EXAMPLE 2 Nitrite Test Strip A roll of raw paper having a with of about 50 cm., with a thickness of the capillary material of about 320 p. (Schleicher and Schull No. 2316") is impregnated with a solution having the following composition:

0.15 g. of l-naphthylamine 0.85 g. of sulfanilic acid 5 g. of tartaric acid I00 ml. ofwater After drying, the band is cut into paper rolls having a width of about 6 mm. The thus-obtained rolls are continuously placed on a transparent film having a thickness of u, and simultaneously an opaque cover film having a thickness of I00 p. is applied as well and sealed together with the transparent film. The latter contains a bulge corresponding to FIG. 2, the height of which bulge is about 600 p. and the breadth of which is about 4 mm. The cover film is attached in such a manner that the center of the bulge is disposed approximately above the center of the capillary material. The capillary material is placed onto the transparent film approximately so'that its lower edge is 4 6 mm. removed from the lower edge of the film.

The projections provided at the bulges of the cover film in accordance with FIG. 2 retain the capillary material in the desired position. After cutting the thusobtained rolls at right angles to the traveling direction, test strips are obtained having a length of about 60 mm. and a width of 6 mm. The lateral edges of the capillary material are exposed.

EXAMPLE 3 Nickel Test Strip A roll of raw paper having a width of about 50 cm., laminated with a thin polyvinylchloride film (thickness 40 I00 t), which paper is a product bearing Schleicher und Schull No. I451 and has a thickness of 330 350 u, is impregnated with the following solution, and then dried:

0.5 g. of dimethylglyoxime 2 g. of sodium acetate 100 ml. of methanol The thus-obtained roll is cut into paper rolls having a width of 6 mm. and then cemented to an embossed film web, containing a bulge according to FIG. 5. The bulge has a height of about 300 p. and a width of about 4 mm., so that respectively I mm. is available for the attachment of the deep-drawn film on the capillary material at the top and atthe bottom. The thus obtained film band is cut transversely, so that test strips are obtained having a length of about 60 mm. and a width of 6 mm.

In this embodiment, the capillary material is exposed on all four edges.

EXAMPLE 4 Chloride Test Strip A paper strip having a width of about 6 mm. (Schleicher und Schull No. 604 thickness about 220 a) is sealed onto the transparent film band described in Example 1. Thereafter, the band is impregnated with a 0.3 percent aqueous potassium chromate solution and dried. Then, the thus-impregnated paper is saturated with a 0.5 percent aqueous silver nitrate solution, and during this process, uniformly distributed silver chromate is formed in and on the raw paper.

Thereafter, the cover film described in Example 1 is employed for sealing the band, in accordance with the procedure set forth in this example. After cutting the band at right angles to the traveling direction, an indicator strip is obtained along the lines of FIG. 1.

EXAMPLE 5 Cholinesterase Test Strip Similarly as in Example 1, an indicator strip is produced wherein the impregnating liquid is obtained as follows:

100 mg. of phenol red, 100 mg. of naphthol phthalein, and g. of acetylcholine hydrochloride are dissolved in 100 ml. of methanol and thereafter adjusted to a pH of 6.0 by the addition of sodium hydroxide solution.

The impregnation can be conducted prior to or after the application of the filter paper to the transparent film. The thus-obtained strips have a length of 60 mm. and a width of 6 mm. The strip serves for the determination of cholinesterase.

The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples.

From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.

What is claimed is:

1. An indicator strip characterized by quick absorption of a liquid medium to be analyzed, comprising at least one absorbent, reagent-impregnated capillary material disposed between liquid-impermeable films, wherein at least one of said films is at least partially transparent, said strip having open ends and said capillary material having its edges exposed at least at said ends, and at least one slot extending longitudinally through said strip to said open ends and disposed adjacent said capillary material.

2. An indicator strip in accordance with claim 1 wherein the said slot extends over at least 40 percent of a surface of the impregnated absorbent capillary material.

3. An indicator strip in accordance with claim 1 wherein said capillary material contacts one of said films on one side, and wherein said slot is betweensaid other film and said capillary material.

4. An indicator strip in accordance with claim 1 wherein said slot is subdivided into several smaller slots.

5. An indicator strip in accordance with claim 1 wherein said capillary material is also exposed on at least one of its lateral edges.

6. An indicator strip in accordance with claim 1 wherein said capillary material is exposed on all of its edges.

7. An indicator strip in accordance with claim 1 wherein the ratio of the thickness of said capillary material to the maximum height of said slot, in cross section, is about 1 :0.5 to about 1 10.

8. An indicator strip in accordance with claim 1 wherein the ratio of the thickness of said capillary material to the maximum height of said slot, in cross section, is about 1 1 to about 1 :2.

9. An indicator strip in accordance with claim 1 wherein said capillary material has a thickness of about 0.05 to 3 mm.

10. An indicator strip in accordance with claim 1 wherein the capillary material is filter paper.

11. An indicator strip in accordance with claim 1 wherein said strip has a length of about 30 100 mm., and a width of about 3 10 mm.

12. An indicator strip in accordance with claim 1 wherein said strip has a length of about 50 mm., and a width of about 5 7 mm.

13. An indicator strip in accordance with claim 1 wherein said capillary material has a surface of about 10 50 mm and an approximately square configuration.

14. An indicator strip in accordance with claim 1 wherein said capillary material has been impregnated in situ.

15. An indicator strip in accordance with claim 1 wherein one of said films is transparent.

16. An indicator strip in accordance with claim 1 wherein one of said films forming said slot has a geometrical configuration.

17. An indicator strip in accordance with claim 1 wherein said film forming said slot has a patterned inner surface.

18. An indicator strip in accordance with claim 1, comprising at least two adjacent indicator strips wherein each of said strips is independent from the other and comprises a said capillary material and a said slot, and each of said strips have common films.

19. An indicator strip in accordance with claim 1 wherein said slot is partially filled by projections extending from the inner side of one of said films.

20. An indicator strip in accordance with claim 19 wherein up to about 60 percent of the volume of said slot is filled by saidprojections.

21. An indicator strip in accordance with claim 1 wherein said capillary material is disposed on one of said films and between hydrophobic separating layers.

22. An indicator strip in accordance with claim 21 wherein the ratio of the thickness of the said capillary material to the thickness of said hydrophobic separating layers is about 1 20.5 to about 1 10.

23. An indicator strip in accordance with claim 21 wherein the ratio of the thickness of the said capillary material to the thickness of said hydrophobic separating layers is about 1 1 to about 1 2.

24. An indicator strip in accordance with claim 21 wherein one of said films is secured to the separating layers.

Claims (23)

1. An indicator strip characterized by quick absorption of a liquid medium to be analyzed, comprising at least one absorbent, reagent-impregnated capillary material disposed between liquid-impermeable films, wherein at least one of said films is at least partially transparent, said strip having open ends and said capillary material having its edges exposed at least at said ends, and at least one slot extending longitudinally through said strip to said open ends and disposed adjacent said capillary material.

2. An indicator strip in accordance with claim 1 wherein the said slot extends over at least 40 percent of a surface of the impregnated absorbent capillary material.

3. An indicator strip in accordance with claim 1 wherein said capillary material contacts one of said films on one side, and wherein said slot is between said other film and said capillary material.

4. An indicator strip in accordance with claim 1 wherein said slot is subdivided into several smaller slots.

5. An indicator strip in accordance with claim 1 wherein said capillary material is also exposed on at least one of its lateral edges.

6. An indicator strip in accordance with claim 1 wherein said capillary material is exposed on all of its edges.

7. An indicator strip in accordance with claim 1 wherein the ratio of the thickness of said capillary material to the maximum height of said slot, in cross section, is about 1 : 0.5 to about 1 : 10.

8. An indicator strip in accordance with claim 1 wherein the ratio of the thickness of said capillary material to the maximum height of said slot, in cross section, is about 1 : 1 to about 1 : 2.

9. An indicator strip in accordance with claim 1 wherein said capillary material has a thickness of about 0.05 to 3 mm.

10. An indicator strip in accordance with claim 1 wherein the capillary material is filter paper.

11. An indicator strip in accordance with claim 1 wherein said strip has a length of about 30 - 100 mm., and a width of about 3 - 10 mm.

12. An indicator strip in accordance with claim 1 wherein said strip has a length of about 50 - 70 mm., and a width of about 5 -7 mm.

13. An indicator strip in accordance with claim 1 wherein said capillary material has a surface of about 10 - 50 mm2 and an approximately square configuration.

14. An indicator strip in accordance with claim 1 wherein said capillary material has been impregnated in situ.

15. AN indicator strip in accordance with claim 1 wherein one of said films is transparent.

16. An indicator strip in accordance with claim 1 wherein one of said films forming said slot has a geometrical configuration.

17. An indicator strip in accordance with claim 1 wherein said film forming said slot has a patterned inner surface.

18. An indicator strip in accordance with claim 1, comprising at least two adjacent indicator strips wherein each of said strips is independent from the other and comprises a said capillary material and a said slot, and each of said strips have common films.

19. An indicator strip in accordance with claim 1 wherein said slot is partially filled by projections extending from the inner side of one of said films.

20. An indicator strip in accordance with claim 19 wherein up to about 60 percent of the volume of said slot is filled by said projections.

21. An indicator strip in accordance with claim 1 wherein said capillary material is disposed on one of said films and between hydrophobic separating layers.

22. An indicator strip in accordance with claim 21 wherein the ratio of the thickness of the said capillary material to the thickness of said hydrophobic separating layers is about 1 : 0.5 to about 1 : 10.

23. An indicator strip in accordance with claim 21 wherein the ratio of the thickness of the said capillary material to the thickness of said hydrophobic separating layers is about 1 : 1 to about 1 : 2.